The present invention relates to a steering device linked to a behavior stabilizing system in vehicles.
In the case of a vehicle spinning or drifting due to excessive speed or poor driving when traversing curves, it becomes impossible to steer the vehicle according to the driver""s intent. Technology to control the braking force and driving force of the vehicle is being developed in order to prevent unstable vehicle behavior such as drifting or spinning. However, the control of the braking force and driving force to stabilize vehicle behavior is effected irrespective of the driver""s will; the problem with this is that the steering feel becomes bad because steering torque is fluctuated on the basis of change in the braking force and driving force.
It is an object of the present invention to provide a steering device that resolves the above-mentioned problem.
A steering device of the present invention for a vehicle, which includes means for transmitting steering torque applied by the driver to wheels, and is constituted such that at least one of braking force and driving force of the wheels can be controlled for stabilization of vehicle behavior, is characterized in that steering torque can be controlled so as to cancel out change in steering torque according to change in at least one of the braking force and driving force due to the control for the stabilization.
With the above constitution, when the vehicle behavior is stabilized by controlling at least one of the braking force and driving force of the wheels, bad steering feel can be prevented by canceling out the change in the steering torque due to the control for the stabilization. The control for the stabilization of vehicle behavior can be performed by using known means.
The steering device of the present invention preferably comprises a steering shaft for transmitting steering torque applied by the driver to the wheels; an actuator for generating torque to be added to the steering torque applied by the driver; a detector for detecting the steering torque transmitted by the steering shaft; means for determining variables corresponding to change in at least one of braking force and driving force due to control for the stabilization; means for storing relationship between the variables and a change in the steering torque; and means for controlling the torque added by the actuator so as to cancel out the change in the steering torque determined from the variables determined and the relationship stored.
With the above constitution, in the case where at least one of the braking force and driving force is controlled in order to stabilize the vehicle behavior, the change in the steering torque based on the change in at least one of the braking force and driving force can be smoothly canceled out by controlling the torque added to the steering torque applied by the driver.
The variables corresponding to the change in at least one of the braking force and driving force due to the control for the stabilization can be determined as the amount of change from the start of the control for the stabilization in, for example, the braking force of each wheel, rotational speed of each wheel, vehicle speed, steering angle, coefficient of friction between the vehicle and the road surface, yaw rate of the vehicle, and so on. The change in the steering torque can be canceled out with better precision as more types of variables are determined.
The relationship between the variables and the change in the steering torque is established so that the change in the steering torque can be determined from the variables determined according to this relationship. The change in the steering torque can be canceled out when the actuator generates torque with an equal magnitude but opposite direction to the change in the steering torque due to the control for the stabilization.
In the steering device of the present invention, it is preferable that, when at least one of the braking force and driving force is controlled so that a moment acts on the vehicle for the stabilization, steering assistance torque with the same direction as that moment is added by the actuator.
In order to stabilize the vehicle behavior by preventing the vehicle from being oversteered and spinning out, a moment directed opposite to the steering direction of the vehicle is caused to act on the vehicle, for example by making the braking force of the outer wheels greater than the braking force of the inner wheels. In this case, the steering torque is changed so as to cause steering in the direction opposite to the moment acting on the vehicle for the stabilization. Such change in the steering torque acting on the driver can therefore be canceled out by adding steering assistance torque having the same direction as that moment by means of the actuator.
In order to stabilize the vehicle behavior by preventing the vehicle from being understeered and drifting, a moment with the same direction as the steering direction of the vehicle is caused to act on the vehicle, for example by making the braking force of the inner wheels greater than the braking force of the outer wheels. In this case, the steering torque is changed so as to cause steering in the direction opposite to the moment acting on the vehicle for the stabilization. Such change in the steering torque acting on the driver can therefore be canceled out by adding steering assistance torque having the same direction as that moment by means of the actuator.
In the case of controlling the braking force and driving force of the wheels for stabilization of vehicle behavior, the present invention provides a steering device which can improve the steering feel by eliminating fluctuations in steering torque due to the control for the stabilization.